The present invention relates to a method for treatment of coal ash (combustion ash of coal), precisely to production of a desulfurizing agent and a soil improver through mixing treatment of coal ash with water, and to a method of desulfurization with the desulfurizing agent in a coal combustor.
Exhaust gas from a coal combustion boiler generally contains from 100 to 2,000 ppm (by volume) of harmful substances such as sulfur oxides. As they cause acid rain and photochemical smog, it is desired to effectively treat them. Heretofore, a dry process such as an activated charcoal process, and a wet process such as a lime-gypsum process have been developed. However, the dry process is problematic in that the ratio of removal of harmful substances could not be increased therein. In the wet process, the ratio of removal of harmful substances is high, but this is problematic in that the treatment of waste water is difficult and the equipment costs and the running costs are high.
To solve the problems, desired is a desulfurization process in which the ratio of removal of harmful substances is high and which can be run at low costs, not producing waste water. Therefore, for desulfurization in coal combustion boilers,  less than 1 greater than  a semi-dry process of spraying slaked lime or its slurry into exhaust gas,  less than 2 greater than  a dry process of atomizing limestone into high-temperature gas in a gas duct, and  less than 3 greater than  a dry process of directly feeding limestone into a combustor have been proposed, apart from the processes mentioned above. When compared with the activated charcoal process and the lime-gypsum process, these processes are good as the equipment costs and the running costs for them are low, but are still problematic in that the ratio of removal of harmful substances therein is not always high.
In particular, the processes  less than 1 greater than  and  less than 2 greater than  are problematic in that the time for contact of exhaust gas with the desulfurizing component could not be prolonged therein. Therefore, the process  less than 3 greater than  of feeding limestone into a combustor, especially into a fluidized-layer or fluidized-bed combustor is now considered good. Specifically, for boiler systems capable of efficiently burning coal therein, for example, circulation-type, coal combustion boilers equipped with a fluidized-bed combustor have been put into practical use, in which the fluidized bed is formed of relatively large-size grains.
The coal combustion boiler of the type equipped with a fluidized-bed combustor is described with reference to a conceptual view showing it. FIG. 1 is a conceptual explanatory view showing a fluidized bed-type, coal combustion boiler for one embodiment of the desulfurization method of the present invention. The fluidized bed-type, coal combustion boiler comprises a fluidized-bed combustor 1 having a fluidized bed 2 in its part; a cyclone 3 for separating particulates from the waste gas from the combustor 1; an external heat exchanger 4 for utilizing the heat of the particulates separated in the cyclone 3; and a convection-type heat-transfer unit 5.
In the combustor 1, the fluidized bed 2 is formed of, for example, gravel of from 10 to 20 mm or so in size; and a desulfurizing agent such as limestone is fed along with coal into the fluidized bed via a feed port 7. Below the fluidized bed 2, air is fed into the combustor via a primary air introduction port 8, with which the coal in the combustor is burned up. Burnt ash and carbon and other small grains of limestone and ground gravel are moved to the cyclone 3. The small grains are separated in the cyclone 3, and drop down into the external heat exchanger 4 disposed below the cyclone 3.
The small grains, of which the heat is recovered in the heat exchanger 4, are circulated back to the fluidized bed 2. The high-temperature gas having been separated from the small grains in the cyclone 3 is led into the convention-type heat-transfer unit 5, in which the gas heats water in the heat-transfer tube 10 to produce steam, and is thereby cooled. Then, the gas is led into a bag filter 6, in which fine particulates in the gas are trapped, and the gas with no particulates is discharged out of the system.
The coal ash thus trapped in the bag filter (ash collector) is referred to as xe2x80x9cbag ashxe2x80x9d, and this fine particulates having a size of a few microns and scattering. In general, therefore, water is added thereto. Thus wetted, this is discarded or utilized for land reclamation, or a part of it is utilized for cement. However, the location of coal combustion boiler plants is not always near to the location for land reclamation or to the location of cement factories, and utilizing the coal ash for land reclamation or for cement is problematic in that the costs for transporting it are high. On the other hand, utilizing the coal ash for other building materials, for example, for cement aggregate and roadbed materials is investigated. However, since the composition of the coal ash is not always constant, the quality control thereof for such building materials is difficult. At present, therefore, the practical use of the coal ash is limited.
In coal combustion boilers, limestone is added to the fluidized bed in the combustor along with coal thereto, for absorbing sulfur dioxide (SO2) from the burned coal. Therefore, the coal ash contains about 10 to 40% by weight of the ash component derived from the limestone. Of the limestone-derived ash component, calcium having trapped sulfur dioxide therein is generally about 30% by weight. In the other part of the ash component, the limestone has absorbed sulfur dioxide only in its surface and has been solidified to be gypsum (CaSO4), but inside the gypsum, the limestone (CaCO3) is heated and converted into quick lime (CaO), or that is, this is kept unreacted.
Therefore, it is undesirable to use the unreacted lime-containing, coal ash for land reclamation as it is, from the viewpoint of natural resources saving and cost reduction. In addition, it is well known that, when water is added thereto, the unreacted quick lime is hydrated to form calcium hydroxide (Ca(OH)2) with suddenly generating great heat and much expanding. Therefore, taking advantage of this knowledge, various methods have been proposed for recycling the unreacted lime-containing, coal ash.
For example, for the fluidized bed-type, coal combustion boiler as in FIG. 1,  less than 1 greater than  Japanese Patent Laid-Open No. 166110/1996 has proposed a method of recycling the combustion ash from the fluidized-bed combustor, which comprises a step of kneading and solidifying the ash having been collected after the cyclone, with water or with water and a cement-type solidifier, a step of grinding the resulting solid into grains, and a step of circulating the grains into the combustor; and  less than 2 greater than  Japanese Patent Laid-Open No. 42614/1997 has proposed a method of recycling the combustion ash from the fluidized-bed combustor, which comprises a first step of wetting the combustion ash having been collected in a bag filter, with mixing and stirring it for hydration to thereby convert it into re-activated ash, a second step of drying the re-activated ash, and a third step of circulating the dried, re-activated ash into the combustor.
On the other hand,  less than 3 greater than  Japanese Patent Laid-Open No. 35827/1986 has proposed a different method of purifying exhaust gas according to a dry lime process, which comprises leading the particulates having been separated from combustion ash in a bag filter into a classifier where the particulates are classified into a group of coarse, large-size particulates containing fly ash, and a group of fine, small-size particulates containing lime particles of which the surface has been compounded with a harmful acid substance to form a shell to cover each particle, then hydrating the lime particles in the group of fine particulates, with steam to break and remove their shells owing to their volume expansion through hydration, to thereby make them have non-reacted lime exposed out of their surface to form recycled lime particles, and circulating the recycled lime particles into exhaust gas.
In these methods, the gypsum layer formed around the lime particles through the reaction of sulfur oxides with lime in the coal ash is broken by hydration of lime, or that is, the hydration of lime is efficiently utilized therein. Accordingly, in these methods, the final desulfurizing agent suitable to the site where it is fed into the coal combustion boiler system and to the unit via which it is fed thereinto is produced and circulated in the system thereby to ensure efficient desulfurization therein and to reduce the coal ash from the system.
However, having investigated and evaluated the capabilities of the desulfurizing agent formed through hydration of coal ash with water or steam, I, the present inventor has found that the degree of desulfurization with the desulfurizing agent is not high. Therefore, it is believed that the desulfurization method for the coal combustion boiler system of using the desulfurizing agent formed through hydration of coal ash with water or steam could not be put into practical use. Accordingly, the present invention is essentially for providing a method of treating coal ash that makes it possible to recycle (circulate) the treated ash as a desulfurizing agent in a coal combustion boiler system, and providing a desulfurization method for a coal combustion boiler system.
To solve the above-mentioned problems in the related art, I, the present inventor has assiduously studied the process of treating the coal ash that contains a limestone-derived component, for making the treated ash recyclable as a desulfurizing agent in the process, and, as a result, has found that the condition for hydration in the process has a significant influence on the capability of the treated ash for desulfurization. On the basis of this finding, the inventor has completed the present invention.
Specifically, the invention includes the following:
1. A method of treating coal ash that contains a limestone-derived component, by mixing it with water, which comprises mixing the coal ash with water on a condition that the coal ash and water undergo a temperature difference therebetween while they are mixed.
2. The method of treating coal ash of above 1, wherein the temperature difference is at least 30xc2x0 C.
3. The method of treating coal ash of above 1 or 2, wherein coal ash at a temperature falling between 80 and 150xc2x0 C. is mixed with water at a temperature falling between 2 and 50xc2x0 C.
4. The method of treating coal ash of above 1 or 2, wherein coal ash at room temperature is mixed with hot water at a temperature falling between 60 and 98xc2x0 C.
5. The method of treating coal ash of any of above 1 to 4, wherein 100 parts by weight of coal ash is mixed with from 20 to 200 parts by weight of water.
6. The method of treating coal ash of any of above 1 to 5, wherein the mixture of coal ash and water has a mean grain size of from 0.1 to 20 mm.
7. A desulfurizing agent comprising the mixture of coal ash and water obtained in the treating method of any of above 1 to 6.
8. A soil improver comprising the mixture of coal ash and water obtained in the treating method of any of above 1 to 6.
9. A method of desulfurization in a coal combustion boiler system, which comprises mixing coal ash that has been separated from a ash collector in the system and contains a limestone-derived component, with water on the condition that the two undergo a temperature difference therebetween while they are mixed, and feeding the resulting mixture that serves as a desulfurizing agent into the coal combustor in the system.
1. A method of desulfurization in a coal combustion boiler system, which comprises mixing coal ash that has been separated from a ash collector in the system and contains a limestone-derived component, with water on the condition that the coal ash at a temperature falling between 80 and 150xc2x0 C. is mixed with water at a temperature falling between 2 and 50xc2x0 C., and circulating the resulting mixture that serves as a desulfurizing agent into the coal combustor in the system.
2. A method of desulfurization in a coal combustion boiler system, which comprises mixing coal ash that has been separated from a ash collector in the system and contains a limestone-derived component, with water on the condition that the coal ash at room temperature is mixed with hot water at a temperature falling between 60 and 80xc2x0 C., and circulating the resulting mixture that serves as a desulfurizing agent into the coal combustor in the system.
12. The desulfurization method of any of above 9 to 11, wherein 100 parts by weight of coal ash is mixed with from 20 to 200 parts by weight of water.
13. The desulfurization method of any of above 9 to 12, wherein the mixture of coal ash and water having been so mixed that the temperature difference therebetween is at least 30xc2x0 C. and having a mean grain size of from 0.1 to 20 mm serves as a desulfurizing agent.